When an object is moved in the air, a vortex of airflow is generated near a surface of the object. As a size of the vortex is increased, the air resistance is increased. It is known that the air resistance is increased in proportion to square of velocity of the movement of an object.
The air resistance against a wheel for vehicle is described below based on the principal given above. A wheel has a disc and a rim. The disc is disposed at an outer end (end on a side opposite to a vehicle body) of the rim in an axial direction of the wheel. A tire is mounted on an outer periphery of the rim.
While the vehicle is running, airflow from the front hits a front end of a rotating tire. The airflow flows rearward through opposite sides of the wheel (outside and inside in the axial direction of the wheel). The airflow flowing along the outside of the wheel is called side airflow. The airflow flowing under the vehicle body along the inside of the wheel (vehicle body side) is called underfloor airflow.
The side airflow flows relatively smoothly along one side surface of the tire and a designed surface of the disc. On the other hand, a vortex is generated from the underfloor airflow. Specifically, when the underfloor airflow passing through the other side surface of the tire reaches a large opening of the wheel, a portion of the underfloor airflow is separated and led into an inside of the opening. Thereby, the vortex is generated. The airflow containing the vortex is pushed out by the next vortex and blown out from openings of the disk through an inner space of the wheel. Then the airflow joins the side airflow, disturbing the side airflow and causing a vortex to be generated.
As mentioned above, an air resistance caused by the vortex is generated against the wheel. Air resistance is also generated by the underfloor airflow's hitting an inner wall surface of the rim in a rear portion of the wheel.
One of the measures to restrain the generation of the vortex may be to reduce an opening area of the openings of the disc, thereby limiting the amount of the portion of the underfloor airflow joining the side airflow. However, the design of the disc may be compromised in this measure. Moreover, since an amount of the air flowing in the inner space of the wheel is reduced, a braking device housed in the wheel may not be sufficiently cooled.
The Patent Document 1 discloses a wheel for vehicle in which the generation of the vortex is restrained and the air resistance is reduced. Specifically, an annular flow straightening member is attached to an end of a rim on a vehicle body side thereof (end on a side opposite to a disc). The flow straightening member is protruded inward from a bead seat portion in a radial direction, covering a portion of an opening of the wheel on the vehicle body side (portion in a vicinity of a peripheral edge of the opening). The flow straightening member has an annular flat surface on the vehicle body side thereof. The annular flat surface is orthogonal to a central axis of the wheel.
In the wheel of Patent Document 1, an underfloor airflow is straightened along the flat surface of the flow straightening member mentioned above, and the separation of the underfloor airflow in the opening is restrained. Therefore, generated amount of the vortex can be reduced, and an amount of the vortex generated by the portion of the underfloor airflow blowing out toward a side airflow can be reduced. Thereby, air resistance against the wheel can be reduced.